Anisotropic optical and electric properties of β-gallium oxide

The anisotropic properties and applications ofβ-gallium oxide(β-Ga_(2)O_(3))are comprehensively reviewed.All the anisotropic properties are essentially resulted from the anisotropic crystal structure.The process flow of how to exfoliate nanoflakes from bulk material is introduced.Anisotropic optical properties,including optical bandgap,Raman and photolumines-cence characters are comprehensively reviewed.Three measurement configurations of angle-resolved polarized Raman spec-tra(ARPRS)are reviewed,with Raman intensity formulas calculated with Raman tensor elements.The method to obtain the Raman tensor elements of phonon modes through experimental fitting is also introduced.In addition,the anisotropy in elec-tron mobility and affinity are discussed.The applications,especially polarization photodetectors,based onβ-Ga_(2)O_(3)were summa-rized comprehensively.Three kinds of polarization detection mechanisms based on material dichroism,1D morphology and metal-grids are discussed in-depth.This review paper provides a framework for anisotropic optical and electric properties ofβ-Ga_(2)O_(3),as well as the applications based on these characters,and is expected to lead to a wider discussion on this topic.

Amorphous gallium oxide homojunction-based optoelectronic synapse for multi-functional signal processing

In the era of accelerated development in artificial intelligence as well as explosive growth of information and data throughput,underlying hardware devices that can integrate perception and memory while simultaneously offering the bene-fits of low power consumption and high transmission rates are particularly valuable.Neuromorphic devices inspired by the human brain are considered to be one of the most promising successors to the efficient in-sensory process.In this paper,a homojunction-based multi-functional optoelectronic synapse(MFOS)is proposed and testified.It enables a series of basic electri-cal synaptic plasticity,including paired-pulse facilitation/depression(PPF/PPD)and long-term promotion/depression(LTP/LTD).In addition,the synaptic behaviors induced by electrical signals could be instead achieved through optical signals,where its sen-sitivity to optical frequency allows the MFOS to simulate high-pass filtering applications in situ and the perception capability integrated into memory endows it with the information acquisition and processing functions as a visual system.Meanwhile,the MFOS exhibits its performances of associative learning and logic gates following the illumination with two different wave-lengths.As a result,the proposed MFOS offers a solution for the realization of intelligent visual system and bionic electronic eye,and will provide more diverse application scenarios for future neuromorphic computing.

Temperature-dependent bias-stress-induced electrical instability of amorphous indium-gallium-zinc-oxide thin-film transistors

The time and temperature dependence of threshold voltage shift under positive-bias stress（PBS） and the following recovery process are investigated in amorphous indium-gallium-zinc-oxide（a-IGZO） thin-film transistors. It is found that the time dependence of threshold voltage shift can be well described by a stretched exponential equation in which the time constant τ is found to be temperature dependent. Based on Arrhenius plots, an average effective energy barrier Eτ stress= 0.72 eV for the PBS process and an average effective energy barrier Eτ recovery= 0.58 eV for the recovery process are extracted respectively. A charge trapping/detrapping model is used to explain the threshold voltage shift in both the PBS and the recovery process. The influence of gate bias stress on transistor performance is one of the most critical issues for practical device development.

Growth properties of gallium oxide on sapphire substrate by plasma-assisted pulsed laser deposition

Gallium oxide was deposited on a c-plane sapphire substrate by oxygen plasma-assisted pulsed laser deposition(PLD).An oxygen radical was generated by an inductive coupled plasma source and the effect of radio frequency(RF)power on growth rate was investigated.A film grown with plasma assistance showed 2.7 times faster growth rate.X-ray diffraction and Raman spectroscopy analysis showedβ-Ga2 O3 films grown with plasma assistance at 500℃.The roughness of the films decreased when the RF power of plasma treatment increased.Transmittance of these films was at least 80%and showed sharp absorption edge at 250 nm which was consistent with data previously reported.

Electronic synapses based on ultrathin quasi-two-dimensional gallium oxide memristor

Synapse emulation is very important for realizing neuromorphic computing, which could overcome the energy and throughput limitations of today's computing architectures. Memristors have been extensively studied for using in nonvolatile memory storage and neuromorphic computing. In this paper, we report the fabrication of vertical sandwiched memristor device using ultrathin quasi-two-dimensional gallium oxide produced by squeegee method. The as-fabricated two-terminal memristor device exhibited the essential functions of biological synapses, such as depression and potentiation of synaptic weight, transition from short time memory to long time memory, spike-timing-dependent plasticity, and spike-rate-dependent plasticity. The synaptic weight of the memristor could be tuned by the applied voltage pulse, number,width, and frequency. We believe that the injection of the top Ag cations should play a significant role for the memristor phenomenon. The ultrathin of medium layer represents an advance to integration in vertical direction for future applications and our results provide an alternative way to fabricate synaptic devices.

Modeling,simulations,and optimizations of gallium oxide on gallium-nitride Schottky barrier diodes

With technology computer-aided design(TCAD)simulation software,we design a new structure of gallium oxide on gallium-nitride Schottky barrier diode(SBD).The parameters of gallium oxide are defined as new material parameters in the material library,and the SBD turn-on and breakdown behavior are simulated.The simulation results reveal that this new structure has a larger turn-on current than Ga2O3 SBD and a larger breakdown voltage than Ga N SBD.Also,to solve the lattice mismatch problem in the real epitaxy,we add a Zn O layer as a transition layer.The simulations show that the device still has good properties after adding this layer.

Contact resistance asymmetry of amorphous indium–gallium–zinc–oxide thin-film transistors by scanning Kelvin probe microscopy

In this work, a method based on scanning Kelvin probe microscopy is proposed to separately extract source/drain（S/D） series resistance in operating amorphous indium–gallium–zinc–oxide（a-IGZO） thin-film transistors. The asymmetry behavior of S/D contact resistance is deduced and the underlying physics is discussed. The present results suggest that the asymmetry of S/D contact resistance is caused by the difference in bias conditions of the Schottky-like junction at the contact interface induced by the parasitic reaction between contact metal and a-IGZO. The overall contact resistance should be determined by both the bulk channel resistance of the contact region and the interface properties of the metalsemiconductor junction.

Structural,Morphological and Electrical Properties of In-Doped Zinc Oxide Nanostructure Thin Films Grown on p-Type Gallium Nitride by Simultaneous Radio-Frequency Direct-Current Magnetron Co-Sputtering

Zinc oxide （ZnO） is one of the most promising and frequently used semiconductor materials. In-doped nanos- tructure ZnO thin films are grown on p-type gallium nitride substrates by employing the simultaneous rf and dc magnetron co-sputtering technique. The effect of In-doping on structural, morphological and electrical properties is studied. The different dopant concentrations are accomplished by varying the direct current power of the In target while keeping the fixed radio frequency power of the ZnO target through the co-sputtering deposition technique by using argon as the sputtering gas at ambient temperature. The structural analysis confirms that all the grown thin films preferentially orientate along the c-axis with the wurtzite hexagonal crystal structure without having any kind of In oxide phases. The presenting Zn, 0 and In elements＇ chemical compositions are identified with EDX mapping analysis of the deposited thin films and the calculated M ratio has been found to decrease with the increasing In power. The surface topographies of the grown thin films are examined with the atomic force microscope technique. The obtained results reveal that the grown film roughness increases with the In power. The Hall measurements ascertain that all the grown films have n-type conductivity and also the other electrical parameters such as resistivity,mobility and carrier concentration are analyzed.

Recent advances in NiO/Ga_(2)O_(3) heterojunctions for power electronics

Beta gallium oxide(β-Ga_(2)O_(3)) has attracted significant attention for applications in power electronics due to its ultrawide bandgap of ~ 4.8 eV and the large critical electric field of 8 MV/cm. These properties yield a high Baliga's figures of merit(BFOM) of more than 3000. Though β-Ga_(2)O_(3) possesses superior material properties, the lack of p-type doping is the main obstacle that hinders the development of β-Ga_(2)O_(3)-based power devices for commercial use. Constructing heterojunctions by employing other p-type materials has been proven to be a feasible solution to this issue. Nickel oxide(NiO) is the most promising candidate due to its wide band gap of 3.6–4.0 eV. So far, remarkable progress has been made in NiO/β-Ga_(2)O_(3) heterojunction power devices. This review aims to summarize recent advances in the construction, characterization, and device performance of the NiO/β-Ga_(2)O_(3) heterojunction power devices. The crystallinity, band structure, and carrier transport property of the sputtered NiO/β-Ga_(2)O_(3) heterojunctions are discussed. Various device architectures, including the NiO/β-Ga_(2)O_(3) heterojunction pn diodes(HJDs), junction barrier Schottky(JBS) diodes, and junction field effect transistors(JFET), as well as the edge terminations and super-junctions based on the NiO/β-Ga_(2)O_(3) heterojunction, are described.

High-temperature annealing of(201)β-Ga_(2)O_(3) substrates for reducing structural defects after diamond sawing

A commercial epi-ready(201)β-Ga_(2)O_(3) wafer was investigated upon diamond sawing into pieces measuring 2.5×3 mm^(2).The defect structure and crystallinity in the cut samples has been studied by X-ray diffraction and a selective wet etching technique.The density of defects was estimated from the average value of etch pits calculated,including near-edge regions,and was obtained close to 109 cm^(-2).Blocks with lattice orientation deviated by angles of 1-3 arcmin,as well as non-stoichiometric fractions with a relative strain about(1.0-1.5)×10^(-4)in the[201]direction,were found.Crystal perfection was shown to decrease significantly towards the cutting lines of the samples.To reduce the number of structural defects and increase the crystal perfection of the samples via increasing defect motion mobility,the thermal annealing was employed.Polygonization and formation of a mosaic structure coupled with dislocation wall appearance upon 3 h of annealing at 1100℃ was observed.The fractions characterized by non-stoichiometry phases and the block deviation disappeared.The annealing for 11 h improved the homogeneity and perfection in the crystals.The average density of the etch pits dropped down significantly to 8×10^(6) cm^(-2).

Exploring heteroepitaxial growth and electrical properties of α-Ga_(2)O_(3) films on differently oriented sapphire substrates

This study explores the epitaxial relationship and electrical properties of α-Ga_(2)O_(3) thin films deposited on a-plane, mplane, and r-plane sapphire substrates. We characterize the thin films by X-ray diffraction and Raman spectroscopy, and elucidate thin film epitaxial relationships with the underlying sapphire substrates. The oxygen vacancy concentration of α-Ga_(2)O_(3) thin films on m-plane and r-plane sapphire substrates are higher than α-Ga_(2)O_(3) thin film on a-plane sapphire substrates. All three thin films have a high transmission of over 80% in the visible and near-ultraviolet regions, and their optical bandgaps stay around 5.02–5.16 eV. Hall measurements show that the α-Ga_(2)O_(3) thin film grown on r-plane sapphire has the highest conductivity of 2.71 S/cm, which is at least 90 times higher than the film on a-plane sapphire. A similar orientation-dependence is seen in their activation energy as revealed by temperature-dependent conductivity measurements, with 0.266, 0.079, and 0.075eV for the film on a-, m-, r-plane, respectively. The origin of the distinct transport behavior of films on differently oriented substrates is suggested to relate with the distinct evolution of oxygen vacancies at differently oriented substrates. This study provides insights for the substrate selection when growing α-Ga_(2)O_(3) films with tunable transport properties.

β-Ga_(2)O_(3) junction barrier Schottky diode with NiO p-well floating field rings

Recently,β-Ga_(2)O_(3),an ultra-wide bandgap semiconductor,has shown great potential to be used in power devices blessed with its unique material properties.For instance,the measured average critical field of the vertical Schottky barrier diode(SBD)based onβ-Ga_(2)O_(3) has reached 5.45 MV/cm,and no device in any material has measured a greater before.However,the high electric field of theβ-Ga_(2)O_(3) SBD makes it challenging to manage the electric field distribution and leakage current.Here,we showβ-Ga_(2)O_(3) junction barrier Schottky diode with NiO p-well floating field rings(FFRs).For the central anode,we filled a circular trench array with NiO to reduce the surface field under the Schottky contact between them to reduce the leakage current of the device.For the anode edge,experimental results have demonstrated that the produced NiO/β-Ga_(2)O_(3) heterojunction FFRs enable the spreading of the depletion region,thereby mitigating the crowding effect of electric fields at the anode edge.Additionally,simulation results indicated that the p-NiO field plate structure designed at the edges of the rings and central anode can further reduce the electric field.This work verified the feasibility of the heterojunction FFRs inβ-Ga_(2)O_(3) devices based on the experimental findings and provided ideas for managing the electric field ofβ-Ga_(2)O_(3) SBD.

Characterization of vertical Au/β-Ga_2O_3 single-crystal Schottky photodiodes with MBE-grown high-resistivity epitaxial layer

High-resistivity β-Ga203 thin films were grown on Si-doped n-type conductive β-Ga203 single crystals by molecular beam epitaxy （MBE）. Vertical-type Schottky diodes were fabricated, and the electrical properties of the Schottky diodes were studied in this letter. The ideality factor and the series resistance of the Schottky diodes were estimated to be about 1.4 and 4.6 x 10^6 %. The ionized donor concentration and the spreading voltage in the Schottky diodes region are about 4 x 10^18 cm-3 and 7.6 V, respectively. The ultra-violet （UV） photo-sensitivity of the Schottky diodes was demonstrated by a low-pressure mercury lamp illumination. A photoresponsivity of 1.8 A/W and an external quantum efficiency of 8.7 x 10%2% were observed at forward bias voltage of 3.8 V, the proper driving voltage of read-out integrated circuit for UV camera. The gain of the Schottky diode was attributed to the existence of a potential barrier in the i-n junction between the MBE-grown highly resistive β-Ga203 thin films and the n-type conductive β-Ga203 single-crystal substrate.

Preparation and NO_2-gas sensing property of individual β-Ga_2O_3 nanobelt

Monoclinic gallium oxide （βGa2O3） nanobelts are synthesized from gallium and oxygen by thermal evaporation in an axgon atmosphere and their NO2 sensing properties are studied at room temperature. Electron microscopy studies show that the nanobelts have breadths ranging from 30 to 50 nm and lengths up to tens of micrometers. Both the x-ray diffraction （XRD） and the selected are electron diffraction （SAED） examinations indicate that β-Ga203 nanobelts have grown into single crystals. Room temperature NO2 sensing tests show that the current of individual β-Ga2O3 nanobelt decreases quickly, and then gently when the NO2 concentration increases from low to high. It is caused by the NO2 molecule chemisorption and desorption processes in the surface of β-Ga2O3 nanobelt.

Study on a novel vertical enhancement-mode Ga_(2)O_(3) MOSFET with FINFET structure

A novel enhanced mode(E-mode)Ga_(2)O_(3) metal-oxide-semiconductor field-effect transistor(MOSFET)with vertical FINFET structure is proposed and the characteristics of that device are numerically investigated.It is found that the concentration of the source region and the width coupled with the height of the channel mainly effect the on-state characteristics.The metal material of the gate,the oxide material,the oxide thickness,and the epitaxial layer concentration strongly affect the threshold voltage and the output currents.Enabling an E-mode MOSFET device requires a large work function gate metal and an oxide with large dielectric constant.When the output current density of the device increases,the source concentration,the thickness of the epitaxial layer,and the total width of the device need to be expanded.The threshold voltage decreases with the increase of the width of the channel area under the same gate voltage.It is indicated that a set of optimal parameters of a practical vertical enhancement-mode Ga_(2)O_(3) MOSFET requires the epitaxial layer concentration,the channel height of the device,the thickness of the source region,and the oxide thickness of the device should be less than 5×10^(16) cm^(-3),less than 1.5μm,between 0.1μm-0.3μm and less than 0.08μm,respectively.

Sub-bandgap refractive indexes and optical properties of Sidopedβ-Ga_(2)O_(3) semiconductor thin films

In this article,we present a theoretical study on the sub-bandgap refractive indexes and optical properties of Sidopedβ-Ga_(2)O_(3) thin films based on newly developed models.The measured sub-bandgap refractive indexes ofβ-Ga_(2)O_(3) thin film are explained well with the new model,leading to the determination of an explicit analytical dispersion of refractive indexes for photon energy below an effective optical bandgap energy of 4.952 eV for theβ-Ga_(2)O_(3) thin film.Then,the oscillatory structures in long wavelength regions in experimental transmission spectra of Si-dopedβ-Ga_(2)O_(3) thin films with different Si doping concentrations are quantitively interpreted utilizing the determined sub-bandgap refractive index dispersion.Meanwhile,effective optical bandgap values of Si-dopedβ-Ga_(2)O_(3) thin films are further determined and are found to decrease with increasing the Si doping concentration as expectedly.In addition,the sub-bandgap absorption coefficients of Si-dopedβ-Ga_(2)O_(3) thin film are calculated under the frame of the Franz–Keldysh mechanism due to the electric field effect of ionized Si impurities.The theoretical absorption coefficients agree with the available experimental data.These key parameters obtained in the present study may enrich the present understanding of the sub-bandgap refractive indexes and optical properties of impurity-dopedβ-Ga_(2)O_(3) thin films.

Influence of white light illumination on the performance of a-IGZO thin film transistor under positive gate-bias stress

The influence of white light illumination on the stability of an amorphous In GaZnO thin film transistor is investigated in this work. Under prolonged positive gate bias stress, the device illuminated by white light exhibits smaller positive threshold voltage shift than the device stressed under dark. There are simultaneous degradations of field-effect mobility for both stressed devices, which follows a similar trend to that of the threshold voltage shift. The reduced threshold voltage shift under illumination is explained by a competition between bias-induced interface carrier trapping effect and photon-induced carrier detrapping effect. It is further found that white light illumination could even excite and release trapped carriers originally exiting at the device interface before positive gate bias stress, so that the threshold voltage could recover to an even lower value than that in an equilibrium state. The effect of photo-excitation of oxygen vacancies within the a-IGZO film is also discussed.

Plasma-Enhanced Atomic Layer Deposition of Amorphous Ga_(2)O_(3) for Solar-Blind Photodetection

Wide-bandgap gallium oxide(Ga_(2)O_(3))is one of the most promising semiconductor materials for solar-blind(200 nm to 280 nm)photodetection.In its amorphous form,amorphous gallium oxide(a-Ga_(2)O_(3))maintains its intrinsic optoelectronic properties while can be prepared at a low growth temperature,thus it is compatible with Si integrated circuits(ICs)technology.Herein,the a-Ga_(2)O_(3) film is directly deposited on pre-fabricated Au interdigital electrodes by plasma enhanced atomic layer deposition(PE-ALD)at a growth temperature of 250°C.The stoichiometric a-Ga_(2)O_(3) thin film with a low defect density is achieved owing to the mild PE-ALD condition.As a result,the fabricated Au/a-Ga_(2)O_(3)/Au photodetector shows a fast time response,high responsivity,and excellent wavelength selectivity for solar-blind photodetection.Furthermore,an ultra-thin MgO layer is deposited by PE-ALD to passivate the Au/a-Ga_(2)O_(3)/Au interface,resulting in the responsivity of 788 A/W(under 254 nm at 10 V),a 250-nm-to-400-nm rejection ratio of 9.2×10^(3),and the rise time and the decay time of 32 ms and 6 ms,respectively.These results demonstrate that the a-Ga_(2)O_(3) film grown by PE-ALD is a promising candidate for high-performance solar-blind photodetection and potentially can be integrated with Si ICs for commercial production.

全氧化镓薄膜同质p-n结

制备p-n结以及探索其物理机制在发展各种功能器件和推进其实际应用中起到关键作用.超宽禁带半导体在制备高压高频器件上有着巨大的潜力,但是氧化镓p型掺杂困难限制了氧化镓同质p-n结的制备,进而阻碍了全氧化镓基双极型器件的发展.本文通过一种先进的相转变生长技术结合溅射镀膜的方法,成功制备了n型锡掺杂β相氧化镓/p型氮掺杂β相氧化镓薄膜.本工作成功制作了全氧化镓单边突变同质p-n结二极管,并且详细分析了器件机理.该二极管实现了4×10^(4)的整流比、在40 V下9.18 mΩcm^(2)的低导通电阻、4.41 V的内建电势和1.78的理想因子,并在交流电压下表现出没有过冲的整流特性以及长期稳定性.本工作为氧化镓同质p-n结初窥门径,为氧化镓同质双极型器件奠定了基础,为高压高功率器件的应用开创了道路.

Strain-enhanced polarization sensitivity inβ-Ga_(2)O_(3)photodetector

Polarization-sensitive photodetection and imaging have great application value in fields such as polarization division multiplexing optical communication,remote sensing,near-field imaging and military monitoring.Pursuing a high polarization ratio has always been the research hotspot in polarization-sensitive photodetectors.In this paper,we report a compression strain enhanced polarization ratio inβ-gallium oxide(β-Ga_(2)O_(3))single crystal flake.A rigorous crystallographic analysis confirmed its high crystalline quality and orientation.Angle-resolved polarization Raman spectroscopy(ARPRS)was adopted to study the anisotropy of its optical properties.Extensive ARPRS measurements and theoretical calculation consistently demonstrate the strong optical anisotropy in the high-qualityβ-Ga_(2)O_(3)flake.A polarization ratio of 0.96 was obtained in the flatβ-Ga_(2)O_(3)flake.Furthermore,mechanical strain of±0.7%was introduced intoβ-Ga_(2)O_(3).An increased polarization ratio of 0.98 was achieved in the case of 0.7%compression strain,which is,to the best of our knowledge,the highest value for UVC polarization-sensitive photodetectors.That corresponds to an improved polarization rejection ratio of 100.This work proposed a new path towards improving polarization sensitivity by applying strain engineering in the active material.